RESPIRATION: THE GASEOUS INTERCHANGES 377 



the blood depends on its red corpuscles, since pure blood-plasma 

 or blood-serum is colorless, or at most a very faint straw yellow. 

 Hence the color change which the blood experiences in circulating 

 through the lungs must be due to some change in its red corpuscles. 

 We have already seen (Chap. XVII) that the functional sub- 

 stance of the red corpuscles is hemoglobin, which has the prop- 

 erty of combining with oxygen. Hemoglobin itself is of a dark 

 purplish color, when combined with oxygen the resulting com- 

 pound is a bright scarlet. Hemoglobin combined with oxygen is 

 known as oxyhemoglobin, and it is on its predominance that the 

 color of arterial blood depends. Hemoglobin uncombined with 

 oxygen, sometimes named reduced hemoglobin, predominates in 

 venous blood, and is the only kind found in the blood of a suffo- 

 cated mammal. 



The Laws Governing the Absorption of Gases by a Liquid. In 

 order to understand the condition of the gases in the blood liquid 

 it is necessary to recall the general laws in accordance with which 

 liquids absorb gases. They are as follows: 



1. A given volume of a liquid at a definite temperature if it 

 absorbs any of a gas to which it is exposed, and yet does not com- 

 bine chemically with it, takes up an amount of the gas which de- 

 pends upon two things: (1) the solubility of the gas in the liquid; 

 and (2) the pressure of the gas upon the surface of the liquid. As 

 the pressure of the gas is increased the amount of it which goes in 

 solution in the liquid is increased in exactly the same proportion. 

 If a complete vacuum be formed above a liquid all the gas con- 

 tained within it is given off. This law, that the quantity of a gas 

 dissolved by a liquid varies directly as the pressure of that gas on 

 the surface of -the liquid is known as Henry's law. 



2. The amount of a gas dissolved by a liquid depends, not on the 

 total pressure exerted by all the gases pressing on its surface, but 

 on the fraction of the total pressure which is exerted by the par- 

 ticular gas in question. For example, the average atmospheric 

 pressure is equal to that of a column of mercury 760 mm. (30 

 inches) high. But 100 volumes of air contain approximately 80 

 volumes of nitrogen and 20 of oxygen; therefore | of the total pres- 

 sure is due to oxygen and to nitrogen.: and the amount of oxygen 

 absorbed by water is just the same as if all the nitrogen were re- 

 moved from the air and its total pressure therefore reduced to of 



